Ian M. Brodie

Suspended particle characteristics in storm runoff from urban impervious surfaces in Toowoomba, Australia

Total suspended solids as a measure of suspended particles in urban stormwater has limitations and the alternative suspended sediment concentration method was adapted to determine non-coarse particle (NCP) concentration, defined as particles smaller than 500 μm. NCP was partitioned into the following classes: very fine particles (<8 μm, VFP), fine particles (8–63 μm, FP) and medium particles (63–500 μm, MP). A site mean concentration approach was adopted to differentiate the suspended particle characteristics between three impervious surfaces (roof, road and car park) using runoff data collected for 35 storms. Runoff particle size distribution (PSD) of all surfaces was dominated by particles less than 63 μm. A weak trend of relatively constant VFP concentration was present in the road runoff data. Roof runoff PSD became finer as NCP concentration increased and, overall, the PSD of car park runoff was coarser compared to road and roof runoff. These findings have runoff treatment implications as settling processes are influenced by particle size.

Hydrological analysis of single and dual storage systems for stormwater harvesting.

As stormwater flows are intermittent, the requirement to store urban runoff is important to the design of a stormwater re-use scheme. In many urban areas, the space available to provide storage is limited and thus the need to optimise the storage volume becomes critical. This paper will highlight the advantages and disadvantages of two different approaches of providing storage: 1) a single shallow storage (0.5 m depth) in which stormwater capture and a balanced release to supply users is provided by the one unit; and 2) a dual system in which the functions of stormwater capture and supply release are provided by two separate deeper storage units (2 m depth). The comparison between the two strategies is supported by water balance modelling assessing the supply reliability and storage volume requirements for both options. Above a critical volumetric capacity, the supply yield of a dual storage system is higher than that from a single storage of equal volume mainly because of a smaller assumed footprint. The single storage exhibited greater evaporation loss and is more susceptible to algae blooms due to long water residence times. Results of the comparison provide guidance to the design of more efficient storages associated with stormwater harvesting systems.

Using Volume Delivery Time to Identify Independent Partial Series Events

In practice, the annual series of streamflow peaks is generally preferred than the partial series for flood frequency analysis. Flood selection criteria for the partial series tend to be arbitrary and are limited in making allowances for catchment scale. This aspect appears to be a constraint to greater acceptance of the partial series approach. The aim of this paper is to define a scalable selection criterion that reduces ambiguity in flood selection by defining floods that exceed the daily average. An approach based on the volume delivery time (VDT), analogous to the tip interval time in tipping bucket raingauges, is described and tested for three rural catchments of various sizes in South East Queensland, Australia. The VDT approach produced discharge quantile estimates similar to the partial series based on the commonly-used monthly maxima except for minor, high frequency discharges at the small, more perennial catchment. A simplified approach based on average daily volume gave similar results to the VDT method.

Using Soil Loss Models to Estimate Suspended Solids Concentrations in Stormwater Runoff from Pre-urban Areas

Abstract Estimation of pollutant concentrations and mass loads in runoff is often required to develop stormwater management plans for future urban areas. Total Suspended Solids (TSS) is an indicator of stormwater pollution. There is often the need to predict and compare “post-urban” and “pre-urban” conditions to quantify the likely future change in TSS loads due to urban development. This paper focuses on planning-level estimation of TSS concentrations and loads from pre-urban sites, specifically rural grazing land. The proposed method is an adaptation of widely-used variants of the Universal Soil Loss Equation and accounts for site specific characteristics, such as climate, topography, vegetation cover and soil type. A case study is provided to demonstrate the application of the proposed method. The method is considered to be an improvement to current methods based on Event Mean Concentration (EMC), or at least provides guidance on the selection of an appropriate TSS EMC value to apply to pre-urban areas.

A direct analysis of flood interval probability using approximately 100-year streamflow datasets

ABSTRACT Series of observed flood intervals, defined as the time intervals between successive flood peaks over a threshold, were extracted directly from 11 approximately 100-year streamflow datasets from Queensland, Australia. A range of discharge thresholds were analysed that correspond to return periods of approximately 3.7 months to 6.3 years. Flood interval histograms at South East Queensland gauges were consistently unimodal whereas those of the North and Central Queensland sites were often multimodal. The exponential probability distribution (pd) is often used to describe interval exceedence probabilities, but fitting utilizing the Anderson-Darling statistic found little evidence that it is the most suitable. The fatigue life pd dominated sub-year return periods (<1 year), often transitioning to a log Pearson 3 pd at above-year return periods. Fatigue life pd is used in analysis of the lifetime to structural failure when a threshold is exceeded, and this paper demonstrates its relevance also to the elapsed time between above-threshold floods. At most sites, the interval medians were substantially less than the means for sub-year return periods. Statistically the median is a better measure of the central tendency of skewed distributions but the mean is generally used in practice to describe the classical concept of flood return period. Editor Z.W. Kundzewicz; Associate editor I. Nalbantis

Comparison of Methods to Estimate Suspended Solids Loads in Urban Stormwater

Abstract Planning of stormwater management strategies often requires estimating the concentration or mass load of pollutants in storm runoff generated from urban areas. In Australia, it is common practice to use simple methods that adopt a constant event mean concentration (EMC) for all storms under analysis. Alternatively, a stochastic concentration may be used to introduce variability between storm events. A comparison is made between three of these “Mean EMC Methods” and five proposed methods. Each method was evaluated in terms of accuracy in predicting suspended solids EMCs against data measured for a road surface located in Toowoomba, Queensland. The cumulative error in estimating the total load for a sequence of storm events was also assessed. The Mean EMC Methods demonstrated low accuracies compared to the proposed alternatives.

SSUIS: a research model for predicting suspended solids loads in stormwater runoff from urban impervious surfaces

Suspended solids from urban impervious surfaces (SSUIS) is a spreadsheet-based model that predicts the mass loading of suspended solids (SS) in stormwater runoff generated from impervious urban surfaces. The model is intended to be a research tool and incorporates several particle accumulation and washoff processes. Development of SSUIS is based on interpretation of storm event data obtained from a galvanised iron roof, a concrete car park and a bitumen road located in Toowoomba, Australia. SSUIS is a source area model that tracks the particle mass balance on the impervious surface and within its lateral drain to a point of discharge. Particles are separated into two groups: free and detained, depending on the rainfall energy required for surface washoff. Calibration and verification of SSUIS against the Toowoomba SS data yielded R(2) values ranging from 0.60 to 0.98. Parameter sensitivity analysis and an example of how SSUIS can be applied to predict the treatment efficiency of a grass swale are also provided.